Schema fixes

The major change here is converting the record-subject relation to many-to-many.  I suspect that the "kytopic" table needs to be in a many-to-many relationship with records, but I also think it needs a different name, and I'd rather have that fixed first to simplify later migrations.

There are various minor tweaks, mostly dealing with adjusting names to adhere to Rails naming conventions or to be more human-readable.

Finally, I added an ERD visualizer.  (If you pull this, you will need to rerun "bundle install" to pick up the appropriate gem.)  This adds the "rake erd" task, which generates an entity-relationship diagram for the app.  By default, it creates the diagram as a PDF in the root.  I've added erd.pdf and public/erd.pdf to make it easier to view the ERD without accidentally committing it to the repository.

:coffee: :penguin: